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PDF RT7251B Data sheet ( Hoja de datos )
| Número de pieza | RT7251B | |
| Descripción | (RT7251A/B) Synchronous Step-Down Converter | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT7251A/B
1.5A, 17V, 340/800kHz Synchronous Step-Down Converter
General Description
The RT7251A/B is a high efficiency, monolithic
synchronous step-down DC/DC converter that can operate
at 340kHz/800kHz, while delivering up to 1.5A output
current from a 4V to 17V input supply. The RT7251A/B's
current mode architecture allows the transient response
to be optimized. Cycle-by-cycle current limit provides
protection against shorted outputs and soft-start eliminates
input current surge during start-up. Fault conditions also
include output under voltage protection, output over voltage
protection and thermal shutdown. The low current (<5μA)
shutdown mode provides output disconnection, enabling
easy power management in battery-powered systems. The
RT7251A/B is available in a WDFN-8L 2x2 package.
Ordering Information
RT7251A/B
Package Type
QW : WDFN-8L 2x2 (W-Type)
Lead Plating System
Z : ECO (Ecological Element with
Halogen Free and Pb free)
Note :
A : 340kHz
B : 800kHz
Richtek products are :
` RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
` Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
RT7251AZQW
03 : Product Code
03W
W : Date Code
Features
z 4V to 17V Input Voltage Range
z 1.5A Output Current
z Internal N-MOSFETs
z Current Mode Control
z Fixed Frequency Operation : 340kHz/800kHz
z Output Adjustable from 0.8V to 12V
z Up to 95% Efficiency
z Internal Compensation
z Stable with Low ESR Ceramic Output Capacitors
z Cycle-by-Cycle Over Current Protection
z Input Under Voltage Lockout
z Output Under Voltage Protection
z Output Over Voltage Protection
z Power Good Indicator
z Thermal Shutdown Protection
z RoHS Compliant and Halogen Free
Applications
z Industrial and Commercial Low Power Systems
z Computer Peripherals
z LCD Monitors and TVs
z Green Electronics/Appliances
z Point of Load Regulation for High-Performance DSPs,
FPGAs, and ASICs
Pin Configurations
(TOP VIEW)
SW 1
VIN 2
BOOT 3
EN 4
8 GND
7 GND
6 PGOOD
9 5 FB
WDFN-8L 2x2
RT7251BZQW
70 : Product Code
70W
W : Date Code
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
Free Datasheet http://www.datasheet4u.com/
1 page  
RT7251A/B
Parameter
Minimum On-Time
Input Under Voltage Lockout
Threshold
Input Under Voltage Lockout
Threshold Hysteresis
EN Threshold
Voltage
Logic-High
Logic-Low
EN Pull Low Current
Soft-Start Period
Thermal Shutdown
Thermal Shutdown Hysteresis
Power Good Threshold Rising
Power Good Threshold
Hysteresis
Power Good Pull Down
Resistance
Output OVP Threshold
Output OVP Propagation
Delay
Symbol
tON
VUVLO
ΔVUVLO
VIH
VIL
tSS
TSD
ΔTSD
Test Conditions
VEN = 2V, VFB = 1V
Min Typ Max Unit
-- 100 --
ns
-- 3.5 --
V
-- 200 --
2.5 --
--
-- -- 0.4
-- 1 --
-- 1 --
-- 150 --
-- 15 --
-- 0.7 --
-- 130 --
mV
V
μA
ms
°C
°C
V
mV
-- 12 --
Ω
-- 125 -- %VREF
-- 10 --
μs
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
Free Datasheet http://www.datasheet4u.com/
5 Page 
RT7251A/B
Application Information
The RT7251A/B is a synchronous high voltage buck
converter that can support the input voltage range from
4V to 17V and the output current can be up to 1.5A.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
VOUT
FB
RT7251A/B
GND
R1
R2
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
VOUT
=
VFB
⎛⎜⎝1
+
R1
R2
⎞
⎟⎠
where VFB is the feedback reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET. It is recommended
to add an external bootstrap diode between an external
5V and the BOOT pin for efficiency improvement when
input voltage is lower than 5.5V or duty ratio is higher
than 65%. The bootstrap diode can be a low cost one
such as 1N4148 or BAT54. The external 5V can be a 5V
fixed input from system or a 5V output of the RT7251A/B.
Note that the external boot voltage must be lower than
5.5V.
5V
BOOT
RT7251A/B
SW
10nF
Figure 2. External Bootstrap Diode
Over Voltage Protection (OVP)
The RT7251A/B provides over voltage protection function
when output voltage is over 125%. The internal MOS will
be turned off. The control will return to normal operation if
over voltage condition is removed.
Under Voltage Protection (UVP)
For the RT7251A/B, it provides Hiccup Mode Under
Voltage Protection (UVP). When the FB voltage drops
below 50% of the feedback reference voltage, the UVP
function will be triggered and the RT7251A/B will shut down
for a period of time and then recover automatically. The
Hiccup Mode UVP can reduce input current in short-circuit
conditions.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current £GIL increases with higher VIN
and decreases with higher inductance.
ΔIL
=
⎡
⎢⎣
VOUT
f ×L
⎤
⎥⎦
×
⎡⎢1−
⎣
VOUT
VIN
⎤
⎥
⎦
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal. For the ripple current
selection, the value of ΔIL = 0.2(IMAX) will be a reasonable
starting point. The largest ripple current occurs at the
highest VIN. To guarantee that the ripple current stays
below the specified maximum, the inductor value should
be chosen according to the following equation :
L
=
⎡
⎢
⎢⎣
f
×
VOUT
ΔIL(MAX)
⎤
⎥
⎥⎦
×
⎡
⎢1−
⎢⎣
VOUT
VIN(MAX)
⎤
⎥
⎥⎦
Table 2. Suggested Inductors for Typical
Application Circuit
C ompo nen t
Supplier
TDK
Ser ie s
VLF10045
D ime nsion s
(mm)
10 x 9.7 x 4.5
TDK
SLF12565 12.5 x 12.5 x 6.5
TAIYO
YUDEN
NR8040
8x8x4
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
Free Datasheet http://www.datasheet4u.com/
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet RT7251B.PDF ] | |
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